What is Living in my Mouth?

Biofilms, Carbohydrates and acid blasts

The last time you opened wide, did you ever imagine that you were opening the door to what is essentially home to thousands of bacteria? Living inside your very own mouth! Yes, hundreds of varieties of critters, eating for free every time you visit your favourite restaurant. The ultimate uninvited dinner guest. Many of these bacteria are fairly benign and exist in your mouth without doing any harm at all, but there are some that have the capacity to do some impressive damage.

In the interest of keeping your attention, I'll dedicate another chat to my favourite villains and their superpowers.

One particular kind of bacteria that is both common and relatively abundant among oral flora is the mutans streptococcus group. These common bacteria are particularly good at existing in the oral environment. When you eat carbohydrates, so do they, which is significant because these carbohydrates are metabolized into acid. This acid can lead to cavities in unsuspecting teeth. This acid works to demineralize teeth, stripping the outer enamel layer of it's strengthening minerals. This usually happens slowly over time as the demineralization front moves deeper into the tooth, through the outer layer of enamel, then even faster through the porous dentine* layer to the core or "pulp" of the tooth where nerves and blood vessels reside. Bacteria can then move through the extent of the carious lesion and may result in a necrotic pulp. Hello root canal! (* - Think of dentine as a layer made of drinking straws in parallel embedded in plaster.)

So, the formula for cavities can be thought of as: acid-producing bugs + carbohydrates + teeth. But that's not the whole picture. There are many factors that determine an individual's susceptibility to cavities, or in dental-speak, "caries". They include the relative amounts of acid-producing, (acidogenic) bacteria**, how often carbohydrates are consumed, and the rate of salivary flow, to name just a few. (** - Such as the Streptococcus mutans group of bacteria.)

Some individuals have the good fortune of having relatively low numbers of pathogenic bacteria, whereas others have gobs of them. This means their mouths are charged and ready to produce lots of acid when given the right fuel (carbohydrates). One interesting study examined a population in Africa that, based on their high mutans strep counts, would have been considered a high-risk group for caries. However, the incidence of caries in this population was disproportionately low because of their diet, which was free of simple carbohydrates.

But jumping on the Atkins bandwagon is not necessarily the answer! If carbohydrates are consumed, say, three times a day with each meal, then there are really only three major "acid-attacks" to worry about. However, sipping tea with sugar throughout the day and then sucking on a few sugary after-dinner mints means a multiplicity of fuel blasts for the acid-producing bacteria! Here, frequency becomes an appreciable problem if you have the right type and amount of bacteria in your mouth.

This frequency problem is often the culprit when a patient with low salivary flow rate presents with a mouthful of cavities. "Xerostomia", or "dry-mouth", can be a side-effect of prescription drugs, smoking, radiation, chemotherapy or result from some diseases. These individuals have less saliva sloshing around to rinse the surfaces in their mouth and wash the acidogenic bacteria and their carb-fuel away. The bacteria can stick around longer, colonize and expand their communities into an acidogenic machine!

To temper their xerostomia, patients may try drinking juices or sodas frequently throughout the day or sucking on candies or mints. Unfortunately, sugar is virtually ubiquitous and many patients end up exacerbating, rather than diminishing, their high caries-risk. Drinking sugar-free liquids frequently, enjoying candy and food with sugar-substitutes, and using a saliva-substitute (a gel-like slop or mouthrinse that can be applied to the inside of the mouth and lasts for hours) may be a better solution.

So wait a minute… who invited these bacteria anyway?

Figure 2: Next time you kiss someone, remember this image, which is an electron microscope close-up of the chains of bacteria (cocci) that live on your teeth.

A newborn's mouth is initially sterile, but can become home to new flora as they pass through the birth canal (I'm not going to pretend I'm not grossed-out by this), get kissed by bacteria-laden relatives, and put their fingers, toes, objects galore, into their mouths (see references 1,2,3 below). One study examining the pattern of infectivity of common acidogenic bacteria from mother to child found that there may even be a "window of infectivity". The bacteria studied colonized the child's mouth at a median age of 26 months (reference 4, below).

Oral bacteria differ among sites in the mouth, even among sites on the same tooth, and the flora shifts as the infant begins to sprout teeth at around 6 months. There is a change in structural environment as hard tissues are exposed (tooth enamel) and bacteria that welcome the change can expand and proliferate in this new environment. Other bacteria that enjoyed the gummy environment may decrease in number and the population balance shifts.

How do these bacteria stick around with spit sloshing around and a big muscular tongue?

The resident microbes don't work alone. Together they form a biofilm, aka plaque, which begins with the interplay of electric charges. Certain bacteria can "stick" to oral structures through electrical interaction. Once their anchorage is established, they can bind to other bacteria floating around in the mouth through interesting appendages similar to the tentacles of an octopus. As this web of interacting bacteria builds, it's capacity to hold more bacteria increases and the plaque thickens as more microbes are added. Disrupting this biofilm with food, saliva, a toothbrush and floss is critical in dismantling the microbial community's ability to proliferate and harm teeth and gums. Floss becomes especially important since some sites are more protected than others from mechanical removal, such as between the teeth and along the gumline.

These bacteria are not your friends! They eat your French rolls without your permission! I suggest you blow them away with a nice toothbrush and floss.

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